检查与LINQ和放C＃VAR空值;实体框架 [英] Checking for null value in c# var with LINQ & Entity framework

问题描述

我是很新的LINQ&安培;实体框架以及在C＃中var关键字，所以请原谅我，如果这听起来像一个'新手'的问题。

I'm quite new to LINQ & Entity framework as well as the var keyword in c# so please pardon me if this sounds like a 'newbie' question.

我有做这样的事情出现问题后检查空值：

I have problems checking for null values after doing something like this:

var entry = myDB.Entries.Where(e => e.Email == entry.Email);

即使当电子邮件不存在于数据库中，项不等同于空。

Even when the email does not exist in the database, entry does not equate to null.

所以不是如果（入门== NULL）我不得不这样做如果（ entry.Count（）。1）检查现有条目之前，我执行我的下一批语句。没有任何理由，为什么变量将不被视为空？

So instead of if (entry == null) i had to do if (entry.Count() < 1) to check for existing Entry before i execute my next batch of statements. Is there any reason why the variable wouldn't be considered null?

推荐答案

在你的榜样，进入将的从不的是空。你所认为的空什么是实际上是一个的IEnumerable<输入方式> 有没有项目

In your example, entry will never be null. What you think of as null is in fact an IEnumerable<Entry> with no items.

如果你想检查是否有至少一个条目，你的标准，你通常做这样的事情：

If you want to check if there is at least one entry with your criteria, you normally do something like:

var entries = myDB.Entries.Where(e => e.Email == entry.Email);
if (entries.Any()) {
    // ...
}

如果您知道会有的最多的一个条目，那么你也可以这样做：

If you know that there will be at most one entry, then you can also do:

var entry = myDB.Entries.Where(e => e.Email == entry.Email).SingleOrDefault();
if (entry != null) {
    // ...
}

这是接近你想象的，但将引发，如果有多个匹配条目的异常。

This is closer to what you imagined, but will throw an exception if there is more than one matching entry.

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